1. Field of the Invention
The present invention relates to an electronic large current switch for a single power circuit, and more particularly to an electronic switch applicable to a vehicle having a single DC power supply.
2. Description of the Prior Arts
The conventional switches typically used to control the power supply to electrical loads of a vehicle, such as lamps, window wipers, etc., are classified into mechanical relay switches and electronic switches.
The conventional relay switch has the disadvantages that the state of the contacts for the relay may be unstable and malfunctions of the contacts may occur due to external vibration or impact. Also, the state of the contacts deteriorates when foreign matter such as dust comes in contact with the contacts or when the contacts are partly corroded. Furthermore, when switching a large current load by such relay contacts, the contacts are apt to stick to each other or harm adjacent electronic instruments due to electric arcs which occur between the contacts.
On the other hand, the conventional electronic switch which uses a transistor as a switching element thereof is generally designed to switch a relatively small current load of approximately 2 to 3 amperes and thus the saturation voltage between the collector and the emitter of the switching transistor becomes a high level of 1 to 2 volts because the base current thereof is several tens to several hundreds of milliamperes in accordance with electrical characteristics such as the parameters or the operating condition of the switching transistor. If this type of electronic switch is used as a controller of the power supply for a vehicle, however, the switching transistor controls a large current load of 10 to 15 amperes supplied to the electrical loads. Thus, the power consumption by the switching transistor becomes relatively high and, accordingly, the switching transistor deteriorates quickly due to heat generation within the transistor. Hence, it is difficult to use the conventional electronic switch in a vehicle having a single power supply.
In order to solve the aforementioned problems of such electronic switch, a method for reducing the power consumption of the switching transistor may be considered. The method is to design a switching circuit with a lower saturation voltage by increasing the base current of the switching transistor. However, if a fixed bias type switching transistor using a base bias resistor is employed, the power consumption by the resistor is very high because of a large current load through the resistor and the high voltage difference across the resistor. Therefore, as the power consumption capacity of the resistor must be set high, the size of the resistor increases which in turn makes it difficult to manufacture the electronic switch small, and it can not be the solution to the problem of power loss.
The method of setting the bias voltage by connecting the bleeder type bias resistor to the switching transistor of a Darlington connection may also be considered, but the heat generation of the switching transistor which controls a large current load can not be reduced and the parameters of the transistor are changed due to the generated heat. Therefore, the use of a heat sink having a large surface area is inevitable to radiate the high heat generated within the switching transistor. However, the use of a heat sink alone is not so effective in handling the heat continuously generated in the transistor and accordingly, the switching transistor may not perform effectively and it is difficult to make the product small.
If the conventional electronic switch comprising the transistor as the switching element and the bias circuit is used as the controller of the power supply for a vehicle having a single power supply, the heat loss within the transistor increases and it is difficult to make the product small.